Quasi-isotropic sandwich core

ABSTRACT

A structural core consisting of upper and lower sheets of a fiberglas or like material. Each sheet defines parallel rows of raised, tetrahedronal-shaped, hollow knobs. The upper sheet is inverted over the lower one so that the edges of the tetrahedrons abut whereby all tetrahedrons on the bottom sheet point up and all tetrahedrons on the upper sheet point down in alternating sequence along the rows. Each sheet is formed by placing it over a male mold having upstanding tetrahedron shaped raised knobs, folding the sheet between the tetrahedrons to produce puckers, applying bands in the fold lines to hold the sheet on the male mold, coating the sheet with an epoxy resin, cutting the bands and removing them, inverting a female mold over the male mold and applying pressure thereto, allowing the resin to cure, then removing the sheet from the molds. The core is then formed by coating all edges of the tetrahedrons on both sheets with a resin-type bonding material, inverting the upper sheet and nesting it in the lower sheet so that the edges of the upper tetrahedrons abut the edges of the lower tetrahedrons and then bonded together, the core is then allowed to cure.

United States Patent Figge [54] QUASI-ISOTROPIC SANDWICH CORE [72]Inventor: Irving E. Figge, 183 Revelle Drive, Newport News, Va. 23602[22] Filed: May 20,1970

[21] Appl.No.: 39,598

Primary Examiner-William A. Powell Anomey-l-larry M. Saragovitz, EdwardJ. Kelly and Herbert Berl Feb. 15, 1972 [57] I ABSTRACT A structuralcore consisting of upper and lower sheets of a fiberglas or likematerial. Each sheet defines parallel rows of raised,tetrahedronal-shaped, hollow knobs. The upper sheet is inverted over thelower one so that the edges of the tetrahedrons abut whereby alltetrahedrons on the bottom sheet point up and all tetrahedrons on theupper sheet point down in alternating sequence along the rows. Eachsheet is formed by placing it over a male mold having upstandingtetrahedron shaped raised knobs, folding the sheet between thetetrahedrons to produce puckers, applying bands in the fold lines tohold the sheet on the male mold, coating the sheet with an epoxy resin,cutting the bands and removing them, inverting a female mold over themale mold and applying pressure thereto, allowing the resin to cure,then removing the sheet from the molds. The core is then formed bycoating all edges of the tetrahedrons on both sheets with a resin-typebonding material, inverting the upper sheet and nesting it in the lowersheet so that the edges of the upper tetrahedrons abut the edges of thelower tetrahedrons and then bonded together, the core is then allowed tocure.

8 Claims, 10 Drawing Figures PATENTEDFEB 15 m2 3.642.566

\ INVENTOR Irv/r2 E. 2 1996 1 #mf 5M ATTORNEYS.

4 PAIENTEUFEB 15 m2 3.642.566

SHEET 2 BF 2 INVENTOR Irv/ 179 6 J iifi/Zi M ATTORNEY 1 QUASI-ISOTROPICSANDWICH CORE The invention described herein may be manufactured, used,and licensed by or for the Government for governmental purposes withoutthe payment to me of any royalty thereon.

This invention relates to a constructional core of a honeycomb type andmore particularly isotropic load-carrying properties.

Constructional cores of the honeycomb type have many uses. One such useis in aircraft construction wherein the core must possessquasi-isotropic load-carrying capabilitiesincluding tension,compression, bending, and torsional rigidity. Conventional cores usuallyconsist of two sheets of material with the spacing means sandwichedbetween the sheets. These cores often lacked structural strength and itwas hard to bond the elements together. Most commonly used cores of thehoneycomb type were capable of carrying loads in only one direction,namely in a perpendicular direction to the face sheets.

The present invention is an improvement over the conventional type coresor panels and is made of but two pieces and lends itself to massproduction due to the simplicity of its manufacture. The construction ofthe invention provides a panel that is capable of load carrying in alldirections due to the novel arrangement of the tetrahedronal knobs whichare bonded together at their edges.

It is therefore a principal object of this invention to provide aconstructional panel that possesses quasi-isotropic properties includingtension, compression, bending and torsional rigidity.

It is another object of this invention to provide a structural panelthat is lightweight and formed of but two sheets of a moldable material.

A further object is to provide a constructional core of two moldablesheets having indrawn knobs of tetrahedronal shape, the tetrahedronsbeing bonded together along their edges.

A still further object is to provide a novel and improved method for themanufacture of a structural panel of two moldable sheets wherein each isintended to provide a series of tetrahedronal shaped knobs, one sheetbeing inverted and nested in the other sheet whereby the edges of thetetrahedrons abut and the edges being bonded together.

A final object is to provide a method of making a structural core whichis lightweight, economical to manufacture and which possesses isotropicload-carrying properties.

Other objects and advantages will be apparent from the followingdescription and drawings in which:

FIG. 1 is a fragmentary perspective view of one of the sheets comprisingthe core;

FIG. 2 is a sectional detail view through one of the tetrahedrons andtaken along line 22 of FIG. 1;

FIG. 3 is a fragmentary top view of a completed core, the upper sheetbeing broken away to expose the lower sheet;

FIG. 4 is a diagrammatic view showing a top tetrahedron nested betweenthree bottom tetrahedrons;

FIG. 5 is a fragmentary perspective view of an edge of the completedcore to illustrate positions of the upper and lower tetrahedrons;

FIG. 6 is a fragmentary side view of the mold used in manufacturing thecore;

FIG. 7 is a similar view of the mold and secured to the male mold byrubber bands;

FIG. 8 is a plan view showing in dotted lines the manner of folding asheet to produce puckers therein;

FIG. 9 is a similar view showing rubber bands in place to secure thesheet on the male mold, and,

FIG. 10 is a fragmentary side view showing the mold closed and therubber bands removed.

Referring in detail to the drawing wherein reference character Idesignates generally one of the sheets used in fabricating the corewhich is indicated generally by 2. Core 2 is composed of two identicalsheets, the upper sheet being indicated by 3 while the lower sheet isindicated by 4. Sheets 3 and 4 are provided with series of upstanding,tetrahedronal showing a sheet to a core having quasishaped, hollow knobs5 and 5a respectively. Knobs 5 and 5a are equal in dimensions and arearranged on sheets 3 and 4 in parallel rows, the knobs in one row beingdisposed on the sheet in offset relation with respect to the knobs ofthe ad jacent rows whereby two sides of all of the tetrahedrons aredisposed in oppositely inclining parallel planes while third sides aredisposed in longitudinal parallel planes, all planes intersecting at theapices of their bases. This may readily be observed by FIG. 4 in whichthree tetrahedrons 5 of lower sheet 4 and one tetrahedron 5a are showndiagrammatically. It will be observed that sides of a tetrahedrons 5 liein an inclining plane P sides b lying in oppositely inclining plane Pand sides 0 in longitudinal plane P as shown. Plane P,, P P intersect atd as shown.

Upper sheet 3 is inverted and nested in lower sheet 4 and bondedthereto, so that the tetrahedrons 5a in the upper sheet 3 pointdownward, while the tetrahedrons 5 of the lower sheet 5 point upwardwhereby the edges of an upper tetrahedron will abut one of the edges ofthree lower tetrahedrons 5 as shown.

Sheets 3 and 4 may be of a suitable material as needed, such asfiberglas, plastic, or even metal. The core 2 may be of any size asneeded and a portion of a completed core is shown in FIGS. 3 and 5.

In FIGS. 6-10 is shown the method of forming one of the sheets 1. Amolding apparatus of a suitable material is used and consists of a malemold 6 having upstanding tetrahedronal-shaped knobs 7. The knobs arearranged on the mold 6 similar to a finished sheet 1 as seen in FIG. 9.A female mold 8 having tetrahedronal-shaped recesses 9 therein comprisesthe other member of the mold apparatus. A sheet 1 is now placed over themale mold 6. Where the sheet material is composed of frangible materialsuch as woven fiberglas or thin layers of plastic or metal. it isnecessary to preposition the material along the contours of one of themold sections in order to prevent material rupture during the moldingoperation. In order to accomplish this prepositioning, the sheet foldedalong a series of longitudinal and diagonal lines such as lines 10 and11 between the rows of tetrahedrons 7 on mold 6. This prepositioningproduces puckers 12 as shown in FIG. 7. Holding bands 13 may be used toassist sheets that will not remain positioned on the mold by placingthem on the fold lines 10 and 11 as seen in FIG. 9. The bands may be ofany available material as they are removed and do not form part of theend product. Elastic bands have been found to be especially useful asthey snap out when cut. Sheet I after being positioned is coated with astiffening material such as an epoxy resin. The bands 13 are cut andremoved and pressure is applied to the mold. The resin is allowed tocure and the sheet removed. Two sheets, 3 and 4 are fabricated in thismanner. The edges of all tetrahedrons 5 and 5a are coated with anadhesive such as an epoxy resin and one sheet is inverted and nested inthe other sheet so that the edges of the tetrahedrons abut, as best seenin FIG. 4. The core is now allowed to cure and removed from the mold. Ifthe sheets are of a metal, welding the edges or other well-knownmetal-working procedures may be employed to join the sheets.

It is apparent from the foregoing that a unique, lightweight, highstrength, easily fabricated sandwich constructional core has beenprovided and one which has quasi-isotropic load-carrying capabilitiesincluding tension, compression, bending and torsional rigidity.

What is claimed is:

l. A structural core comprising a pair of substantially identical sheetwalls, each sheet being provided with a series of symmetrically disposedindrawn knobs of tetrahedronal shape the bases of which are separated bytriangular portions of the sheet wall, one sheet being inverted andnested in the other sheet so that the edges of said tetrahedronal knobsof said inverted sheet abut the edges of the tetrahedronal knobs of theother sheet and the triangular sheet wall portions lying betweenadjacent knobs form top and bottom surfaces of the core.

2. The core defined in claim 1 wherein said tetrahedronal shaped knobsin both said sheets are hollow.

3. The core defined in claim 1 wherein said tetrahedronalshaped knobsare arranged on each sheet in longitudinal rows, each row of knobs beingin offset sequential relation to the knobs of an adjacent row wherebytwo sides of all tetrahedrons are disposed in oppositely inclining,parallel planes and the remaining sides are disposed in parallellongitudinal planes, all planes intersecting at the apices of the basesof said tetrahedrons.

4. The core defined by claim 1 wherein the edges of the invertedtetrahedronal-shaped knobs are bonded to the edges of the upstandingtetrahedronal knobs.

5. The core defined by claim 1 wherein all of the saidtetrahedronal-shaped knobs on both sheets are of the same height wherebysaid sheet walls are disposed in spaced, paral le] planes.

6, The core defined by claim 1 wherein said tetrahedronal knobs of onesheet point upwards while the tetrahedronal knobs ofthe other sheetpoint downwards.

7. The core of claim 1 in which the tetrahedronal-shaped knobs of eachsheet are arranged in staggered rows with the bases of the knobs intip-to-tip relation to the bases of adjacent knobs whereby the sides ofthe knobs are disposed in continuous intersecting planes.

87 The core of claim 1 in which the tetrahedronal-shaped knobs of eachsheet are arranged in staggered rows with the triangular portions of thesheet wall separating the knobs being disposed in tip-to-tip relationwith each other to form closed exterior plane surfaces for the core

1. A structural core comprising a pair of substantially identical sheetwalls, each sheet being provided with a series of symmetrically disposedindrawn knobs of tetrahedronal shape the bases of which are separated bytriangular portions of the sheet wall, one sheet being inverted andnested in the other sheet so that the edges of said tetrahedronal knobsof said inverted sheet abut the edges of the tetrahedronal knobs of theother sheet, and the triangular sheet wall portions lying betweenadjacent knobs form top and bottom surfaces of the core.
 2. The coredefined in claim 1 wherein said tetrahedronal shaped knobs in both saidsheets are hollow.
 3. The core defined in claim 1 wherein saidtetrahedronal-shaped knobs are arranged on each sheet in longitudinalrows, each row of knobs being in offset sequential relation to the knobsof an adjacent row whereby two sides of all tetrahedrons are disposed inoppositely inclining, parallel planes and the remaining sides aredisposed in parallel longitudinal planes, all planes intersecting at theapices of the bases of said tetrahedrons.
 4. The core defined by claim 1wherein the edges of the inverted tetrahedronal-shaped knobs are bondedto the edges of the upstanding tetrahedronal knobs.
 5. The core definedby claim 1 wherein all of the said tetrahedronal-shaped knobs on bothsheets are of the same height whereby said sheet walls are disposed inspaced, parallel planes.
 6. The core defined by claim 1 wherein saidtetrahedronal knobs of one sheet point upwards while the tetrahedronalknobs of the other sheet point downwards.
 7. The core of claim 1 inwhich the tetrahedronal-shaped knobs of each sheet are arranged instaggered rows with the bases of the knobs in tip-to-tip relation to thebases of adjacent knobs whereby the sides of the knobs are disposed incontinuous intersecting planes.
 8. The core of claim 1 in which thetetrahedronal-shaped knobs of each sheet are arranged in staggered rowswith the triangular portions of the sheet wall separating the knobsbeing disposed in tip-to-tip relation with each other to form closedexterior plane surfaces for the core.